A “stability of an electric grid” is understood to mean a state in which certain values that characterize the voltages provided by the grid (for example a voltage level, a phase relationship and/or a frequency) are within a range which is defined—possibly by a grid or system operator.
This “stability of an electric grid” defined in the above paragraph can be adversely affected by deviations from a defined desired state, also referred to as an optimum state, wherein the desired state can be described by defined setpoints, e.g. for the above-mentioned characterizing values. Grid faults also constitute such deviations, as they cause a grid state in which one or more of the values designated above deviate from the setpoints.
The grids in question for supplying consumers with electrical energy usually have different grid levels (also referred to as supply levels): A low-voltage level (LV), a medium-voltage level (MV) and a high-voltage level (HV). Expressed simply, these are individual grids with different voltage ranges that are coupled by means of transformer devices.
Grid faults—and deviations from the optimum state in general—can occur at any grid or supply level. To differentiate between individual types of deviation or grid faults, these can be divided into different classes, in particular to differentiate between symmetrical (for example a three-phase voltage dip in a three-phase system) and asymmetrical deviations or faults (for example a single-phase voltage dip or short circuit in a polyphase system, a polyphase short circuit to ground (PE potential) or a short circuit between individual phases).
Grid regulations exist in numerous countries for different grid levels. Requirements for power generation systems (for example for inverters) are defined in these grid regulations, for example, requirements that specify a certain behavior of a power generation system when a certain grid fault occurs, in particular with the objective of supporting the grid.
In the event of a deviation from the optimum state and, in particular, in the event of a grid fault, the decentral power generators can remain connected to the grid and contribute to supporting or stabilizing the grid by purposefully feeding-in active and reactive power. When, in the case of a severe voltage dip, the supporting infeed only takes place for a short time and very dynamically (e.g. in the millisecond range), this is also referred to as “Dynamic Grid Support” (DGS) or “Fault Ride Through” (FRT). The specific embodiments of these requirements in different countries or regions or defined by different grid operators sometimes differ considerably from one another and sometimes even contradict one another. One reason for this is different opinions regarding the nature of an optimum grid support.
One point of agreement between a number of grid regulations is to define a fault event that is to be supported by the generation system as short (considerably shorter than one second). A long-term asymmetry, for example due to a relatively high load on a single phase of a polyphase system, is allowed within certain limits and is not looked upon as being a fault, particularly at the low-voltage level (LV). However, such a case also constitutes a deviation from the optimum state that can escalate into a fault if the values that characterize the grid deviate too greatly from the setpoints. A stabilization of the grid with respect to such deviations is therefore also desirable and is referred to as “static grid support”.
A further point of agreement between a number of grid regulations is that droop functions that define compensation currents to be supplied as a function of the deviations of the prevailing grid voltages from reference values are specified. These so-called reactive current droop characteristics sometimes include dead bands within which no infeed of compensation currents is necessary.
In order to analyze a grid state, the voltages of the individual phases are measured (i.e. the relevant values that describe the voltage—voltage value, phase relationship and possibly frequency—are measured) and transformed into symmetrical components using mathematical methods that are known to the person skilled in the art (for example by means of known matrix operations).
As a result of this mathematical calculation, information relating to a so-called “positive sequence system”, a so-called “negative sequence system” and possibly a so-called “zero sequence system” is obtained. These terms and examples of their calculation using a Fortescue matrix are described in more detail for example in relevant textbooks (e.g. see Heuck et. al. “Elektrische Energieversorgung”, Vieweg Verlag, 7th edition 2007, Chapter 9 & 10) and are known to the person skilled in the art.
If a grid is in a fault-free state and if no asymmetrical loads are present, then the calculations yield no negative sequence system. A purely symmetrical fault only effects a change in the amplitudes of the positive sequence system. On the other hand, an asymmetrical fault and/or an asymmetrical load distribution gives rise to the occurrence of negative sequence system components. Fundamentally, a zero sequence system does not exist in a three wire grid. These basic principles are also known to the person skilled in the art.
Devices and methods for grid support or grid stabilization are also known per se. Classical “grid supporters” are implemented in the form of synchronous machines (SM). Due to their design, synchronous machines generate compensation currents whenever a negative sequence system exists and are therefore able to support or stabilize the grid in that, in the event of grid voltage changes—e.g. caused by grid faults—they generate compensation currents, which oppose the change and therefore damp the grid fault.
A method for wind turbines (WT), which feeds in asymmetrical compensation currents in the event of a grid fault, is disclosed in DE 10 2007 005 165 A1. Along with grid support, the main objective here is to “moderate” the effects of the grid fault on the operation of the WT, thus enabling it to continue operation and not have to be disconnected from the grid. The proposed method essentially comprises tracking a positive sequence system component and a negative sequence system component of the grid state, aligning the negative sequence system component to compensate for asymmetries (asymmetrical grid fault), and feeding-in of compensation currents.
As well as system protection, this method is designed to support the medium-voltage level, wherein the WT is not usually incorporated in a low-voltage grid for supplying electrical consumers and therefore the presence of at least one transformer between the location of a fault and a generator is assumed. A permissible long-term asymmetry as described above (for example a relatively high load on a single phase of a polyphase system) generates a quasistatic negative sequence system at the low-voltage level (LV). The method proposed in DE 10 2007 005 165 A1 cannot be used optimally at the low-voltage level (LV), as in all cases compensation currents are fed in when a negative sequence system is present. On the one hand, these currents are not necessary with imbalances of the voltage amplitudes of the individual phases at the low-voltage level that are permissible per se and, under certain circumstances, can even amplify an asymmetry in a low-voltage supply grid. Furthermore, this method is not able to support symmetrical grid faults due to the exclusive alignment towards the negative sequence system components.
A method for regulating the negative sequence system of a wind turbine (WT), which normally minimizes the active component in the negative sequence system and maximizes its reactive component, is disclosed in DE 10 2006 054 870 A1. A grid fault detector is also provided. In the event of a grid fault, different regulation objectives are pursued with the help of a priority module, i.e. either the active power is maximized (“system protection”) or the active and reactive components of the positive sequence and negative sequence system are optimized in order to support the grid. In doing so, the optimization objective is determined by means of previously defined setpoints of an active or reactive power to be fed in.